1. Field of the Invention
The present invention is related to a thermoroll and an insulation assembly for insulating a heating medium channel running within a shell and/or a shaft of a thermoroll against the conduction of heat. The assembly may be mounted at the end of an oil channel to provide thermal insulation of a shell end area of a thermoroll and/or mounted at the bearing point of a thermoroll shaft by means of a specially designed thermal insulation element.
2. Description of the Related Art
Paper manufacture and finishing processes employ various heatable rolls which are used to remove moisture from the web and particularly to modify the web surface qualities. The greatest mechanical load is imposed on rolls of different types of calendars as the nip pressure of modern equipment is rather high. To obtain higher throughput, which requires wider and faster calendars, the equipment must be dimensioned for extremely high loads. A particularly high stress is imposed on the bearings of heatable rolls, or thermorolls, as these bearings are subjected to the heating resulting from the conduction of heat from the shaft of the roll heated with a heating medium.
Heatable thermorolls are particularly used in softcalenders. As softcalenders are used in on-machine configurations, they must be operated at the same running speed as the paper machine itself. Such a calender has an even number of nips comprising a soft polymer-covered roll and metal-covered thermoroll. The even number of nips, and correspondingly, soft and metal-covered rolls, results from the fact that each polymer-covered roll can be run with one calender nip only, as the roll cannot take the deformations and temperature increase imposed by two nips. Hence, an even number of nips are required to achieve a symmetrical two-sided glaze finish of the web. As board is frequently calendered for higher finish on one side only, a single pair of rolls will be sufficient therein. The temperature of the soft polymer-covered roll must be monitored accurately, and even at a web break, it may not be allowed to touch the surface of the hot thermoroll.
The thermoroll is most commonly heated by hot oil, and in some cases, using other suitable heat transfer media such as water or steam. The heated oil is passed to the interior of the roll via the roll end through a longitudinal bore and then distributed to radial borings in the end flange of the roll, wherefrom the oil further passes to longitudinal borings in the shell of the roll. The circulation of the oil in the roll shell is arranged so that the oil first passes to the opposite end of the roll and then returns therefrom along a parallel boring to the same end by which it was introduced to the shell. The return oil is routed via the end flange and a second bore in the roll shaft back for heating.
The surface of the thermoroll is heated to a rather high temperature in order to impose an intense heat effect on the fast moving web during the short dwell time of the web in the nip. When oil is used for heating the roll, the surface temperature of the roll can be elevated above 200.degree. C. Herein, the temperature of the heating oil passed into the roll may be as high as 280-300.degree. C., which obviously results in an extremely severe heat load on the bearing. Due to the high loads imposed on the roll bearings, the roll shafts must be provided with large bearings, and in fact, the inner diameter of bearings in modern equipment is approximately 0.5 m with an outer diameter of approximately 1 m. Additional loading of the bearings is imposed by the heating caused by the heat flow from the hot oil passed via the roll shaft.
As the price of a bearing increases rapidly when a bearing of higher load rating and diameter is used, the cost impact of the bearing choice is extremely strong. Moreover, the constraints of bearing selection are dictated by the roll diameter, since the bearing and its housing must obviously fit in the space delineated by the thermoroll and the shaft of the polymer-covered roll. If the bearing load becomes so high that the computed diameter of a required size bearing would exceed the space available at the roll end, the bearing load must be reduced by using cooled bearings. This, however, makes the cost of the structure high due to required coolant circulating and cooling machinery. The coolant circulation of the bearing can be connected to the circulating oil system of the paper machine, or alternatively, the calender can be provided with a separate oil circulating system in which circulating oil of higher viscosity can be used.
The length of the thermoroll and the soft roll is slightly larger than the width of the web being finished, whereby noncontacting areas with the web remain at the ends of the rolls. As an extremely high heat flow per unit area is passed from the thermoroll to the web, while at the ends of the roll, heat can leave the roll surface only through radiative and convective losses, whereby the surface temperature of the noncontacting end areas rises higher than that of the roll surface portion making contact with the web. Such temperature difference causes several disadvantages.
The higher temperature at the ends of the roll obviously results in more extensive thermal expansion at the roll end areas. Then, the outer edge of the roll shell bulges outward from the center axis of the roll, and the bending moment caused by thermal expansion correspondingly pushes the area adjacent to the bulging area radially inward toward the center axis. This inward shrunken area coincides exactly with the web edge. Consequently, as the roll diameter at this shrunken area is smaller than at the roll middle, the caliper of the web will be thicker at the web edge than at the web middle. Hence, the cross-machine caliper profile of the web will not be even, which later causes printing problems and thus deteriorates web quality. Moreover, a thick web edge causes difficulties in winding as the wound roll will not be homogeneously tight, and narrower rolls cut from such rolls will be conical. An additional risk is imparted by the hazard of the thermoroll shell contacting the soft roll surface, whereby the polymer covering of the soft roll will be thermally destroyed.
If the thermal expansion of the roll end areas is extensive, the radially expanded end area of the roll may extend so far toward the roll middle as to reach the web edge, whereby a smaller-diameter area of the roll is formed closer to the roll middle, and thus, the web profile will become wavy. As softcalenders are used for finishing large quantities of low-weight paper grades, the web caliper may be extremely thin and even very small variations in the roll diameter and cross-machine contour profile will cause very large relative variations in the cross-machine profile of the paper web. For such reasons, any deformations in the shape of the thermoroll should be kept as smooth and small as possible.
Attempts have been made to alleviate the heating effect of the heating oil passed via the roll shaft by means of a ventilated air gap or thermal insulation. The air gap is formed by an open space about the oil ducts, and such space surrounding the oil supply and return channels of the roll is arranged to communicate with the ambient air in order to ventilate the air gap. However, while the thermal insulation capability of an air gap admittedly is good, such an arrangement has several drawbacks. The greatest problem is caused by heating oil leakage into the gap where it undergoes thermal decomposition forming oil smoke and difficult-to-remove charring which may plug the air gap. Due to the smoke formation, the air gap must be connected to a ventilation duct or exhaust fan with a discharge outside the factory hall. This arrangement may solve the problems caused by the smoke formation, but ventilation of the air gap does not reduce charring. Due to charring, the benefit offered by the air gap remains smaller than expected as the thermal insulation capability of a plugged air gap is inferior. The problem of charring is extremely difficult to overcome, because the end of the thermoroll has a complex oil channel system whose sealing to prevent any oil leakage to the air gap is extremely laborious.
Besides and instead of the air gap, a reduction of heat conduction to the noncontacting end area of a thermoroll has been attempted by means of bushings or coatings made from materials of low thermal conductivity. Coating materials employed have included zirconium oxide, and thermal insulation bushings have been made from polytetrafluoroethylene (PTFE). Obviously, thermal insulation bushings and coatings can be made from a variety of materials such as ceramics and polymer materials. The manufacture of such thermal insulation bushings and coatings is relatively easy and they can be readily adapted about the heating oil channels. However, thermal insulations made from solid materials cannot provide sufficiently good thermal insulation as can PTFE. which, for instance has a thermal conductivity that is ten times the thermal conductivity of air. Because the thermal insulation employed must tolerate a temperature as high as 300.degree. C. under a simultaneous mechanical load, conventional thermal insulation materials cannot be used without the penalty of using an excessively complex structure for the roll shaft.
Finnish patent publication 72,580 discloses a heated roll in which hot oil is circulated in a cavity formed between a cylindrical outer shell of the roll and inner shell adapted concentrically enclosed by the outer shell. The heating medium is passed to the heating cavity via holes made in the end of the inner shell. In this roll embodiment, prevention of excessive heating of the roll end area has been attempted by means of surrounding the entire end of the inner shell with a thermally insulating ring which may be made from a solid material such as polytetrafluoroethylene or manufactured into a hollow, filled metal ring having either a sealed structure, or alternatively, provided with openings, whereby a heat transfer medium can be circulated in the ring. Such a thermal insulation assembly as that disclosed in this patent publication is suited for use only in conjunction with the roll described in the publication, and moreover, has a relatively complex structure which is difficult to manufacture.